I'm not exactly sure what these are called but I searched wall indent and did find some hits on Houzz. [https://www.houzz.com/photos/wall-indent-phbr0lbl-bl~l_48111[(https://www.houzz.com/photos/wall-indent-phbr0lbl-bl~l_48111). I think the middle one was meant for a statue. Regardless, some nice artwork in them would keep with your modern vibe.

the one above the fireplace would need lights to shines from above as shown in the pic below. and some art objects can look really good. or another option is just to fill it in and get rid off it totally and have a painting above the fireplace

the second one, i can imagine would make a great bookshelf if you add a couple of shelves. or you can use the shelves to arrange beautiful objects such as pottery and etc

and the third would be great with a vase with some tall flowers that would add texture

to be honest i would fill the ones in over the fire place. if that is not an option than i would get a two canvas prints made to cover them.

the one in the foyer i would maybe put up a picture on either side, or a key rack, maybe a dish or something to hold mail... make it an entry feature. paint the back of it an accent color

the one in the hall way put two pictures stacked and a vase with flowers.

The niches over the fireplace need to be eliminated, covered with drywall, so you can decorate the mantel properly. Don't try hanging art over top of them; it will be way too high. Anything else you put up there to cover them, say an antique door, will be way too big and overwhelm the fireplace anyway. Best thing is to make them go away.

For the other niches, you could do something like this:

East Lakeview Residence · More Info

I think if you're going to give something a place of honor in a niche, it ought to be something at least a bit out of the ordinary. Otherwise. I'd rather see a blank wall.